1. Field of the Invention
The present invention generally relates to a surface acoustic wave element and a surface acoustic wave device including the surface acoustic wave element, and more particularly, to a surface acoustic wave element that can avoid corrosion at the time of dicing, and a surface acoustic wave device including the surface acoustic wave element.
2. Description of the Related Art
Surface acoustic wave (SAW) devices for filters and resonators utilizing SAW elements are characteristically small in size and inexpensive, and therefore, are widely used for the band-pass filters of portable telephone devices, or the like.
Along with mobile communication devices such as portable telephone devices with higher performances, there is an increasing demand for higher-performance SAW filters. Especially, duplexers and antenna filters are expected to be smaller and to have higher power durability.
So as to achieve higher power durability, the interdigital transducers of a SAW filter are normally made of an aluminum (Al) alloy that has a small density and has a low electrical resistance. In a duplexer or an antenna filter, for example, an aluminum alloy formed by adding an element, such as titanium (Ti), lead (Pb), copper (Cu), niobium (Nb), or magnesium (Mg), to aluminum, is employed. Japanese Unexamined Patent Publication No. 5-206776 discloses a structure that employs such an aluminum alloy. Also, a multi-layer film that is formed by combining aluminum (Al) with another metal may be employed to reduce insertion loss. Example structures that employ such a multi-layer film are disclosed in Japanese Unexamined Patent Publication Nos. 7-122961 and 6-350377.
Conventionally, a SAW element is electrically connected to a package by a wire bonding technique utilizing wires made of aluminum (Al), or the like. In recent years, however, a flip-chip mounting technique utilizing bumps or soldering balls is often employed so as to produce small-sized SAW devices. Japanese Unexamined Patent Publication No. 7-99420 discloses a structure that employs such a flip-chip mounting technique.
FIG. 1 illustrates a conventional SAW element 100. As shown in FIG. 1, the SAW element 100 is a two-stage ladder SAW filter that has four one-port resonators 110 formed on a device substrate (a piezoelectric substrate) 102 made of a piezoelectric material such as lithium tantalate (hereinafter referred to as LT), lithium niobate (hereinafter referred to as LN), or crystal quartz. Each of the resonators 110 includes an IDT 111 and two reflectors 112 sandwiching the IDT 111 in the SAW propagating direction. Two of the four resonators 110 are arranged in a series arm, and the other two of the four resonators 110 are arranged in a parallel arm with interconnections 114.
The IDTs 111 and the interconnections 114 are made of the same electrode material and are integrally formed. Also, electrode pads 113 for inputting and outputting signals are made of the same material as the IDTs 111, and are integrally formed with the IDTs 111. The electrode material may be an aluminum alloy formed by adding approximately 1% of copper (Cu) to aluminum (Al). If the SAW filter 100 is produced as a 1.9 GHz band filter, for example, the film thickness of each IDT 111 is approximately 180 nm. However, the film thickness can be varied with the frequencies of SAW filters. In the above structure, a metal film (such as a film made of gold (Au)) with low electric resistance is laminated on the joining surface (or the bonding surface) of each electrode pad 113, so as to increase the strength of each electrode pad 113 to which wires, bumps, or soldering balls, are connected. Specifically, wettability of solder can be improved in putting gold (Au) film onto bonding pad 113.
In a case where a gold film is laminated on an electrode pad, however, the electric resistance between the joining surfaces increases with time, because intermetal bonding occurs between aluminum (Al) and gold (Au). As a result, the power durability decreases, and the insertion loss increases.
So as to solve this problem, a metal film made of titanium (Ti), chromium (Cr), or the like, is formed as a barrier metal between an aluminum film and a gold film, so that direct bonding of aluminum (Al) and gold (Au) can be prevented. Especially, if titanium (Ti) or chromium (Cr) is employed, the adhesive strength between aluminum (Al) and gold (Au) can be effectively increased.
As described above, so as to increase power durability, a single-layer film formed with an aluminum alloy or a multi-layer film including a film containing aluminum (Al) as the main component and a film containing another metal as the main component is formed at least on each electrode pad 113 on the piezoelectric substrate 102.
In a case where a single-layer film or a multi-layer film containing aluminum (Al) and another metal is employed, however, corrosion occurs due to a galvanic effect between aluminum (Al) and the other metal in cutting water (normally pure water) when a wafer of a multiple-chip structure is diced to obtain SAW elements. The reason of this is that the standard electrode potential of aluminum (Al3+, to be more specific) is −1.66 V, which is much lower than the standard electrode potential of the other metal such as copper (Cu: the standard electrode potential of Cu2+/Cu+ is +0.34 V/+0.52 V). With such a low standard electrode potential, aluminum (Al) is ionized, or melts, in the presence of any of most other metals.
So as to solve this problem, an organic film may be applied by spin-coating in the process immediately before dicing, or a protection film such as an insulating film made of silicon oxide (SiO2) or silicon nitride (SiN) may be formed on each IDT to be protected from corrosion by a film forming technique such as sputtering or vapor deposition.
However, the characteristics of a SAW device greatly vary with the conditions of its surfaces. Especially, in a case where a heavy film is applied, the insertion loss increases, and the filter characteristics deteriorate. Therefore, in the case where an organic film is added so as to prevent the galvanic effect at the time of dicing, the step of removing the organic film from each chip with cleansing liquid needs to be carried out after the dicing step, and the production process becomes more complicated. As a result, the production cost per chip increases.
In the case where an insulating film is formed as a protection film on each IDT, the insulating film has greater influence on the SAW element in a higher frequency band. So as to restrict adverse influence, the insulating film needs to be made as thin as possible. With a thin insulating film, however, holes might appear in the insulating film, and the coverage ratio of each IDT might decrease. If each IDT is not completely covered with an insulating film, corrosion might occur through contact with water. So as to avoid this, the insulating film needs to be thick, though a thick insulting film adversely affects the filter characteristics to a certain extent.